Carbon-carbon brake discs are widely used on commercial and military aircraft. Wide-bodied commercial jets required improved brake materials because traditional steel brake systems simply could not absorb all of the thermal energy created during stops associated with landings. Carbon-based composites were developed which provide heat capacity, thermal conductivity, and thermal strength able to meet the demanding conditions involved in landing big jets. On the military side, the lower weights as well as the thermal and strength properties of the carbon composites has helped to ensure their acceptance in brake applications.
The production of carbon-carbon composite materials, including brake friction materials, has been described extensively in the prior art. One commonly used production method comprises molding a carbon fiber composite with a carbonizable resin, e.g. a phenolic resin, carbonizing the composite “preform”, and then densifying the resulting porous material using chemical vapor infiltration (CVI) and/or resin impregnation processes. Another method comprises building up a fiber preform with textile materials and subsequently densifying the preform using a CVI process. Different structural types of carbon (graphitic, glassy, and pyrolytic) comprise the brake disc, which is somewhat porous. Further densification can be accomplished with, e.g., furfuryl alcohol infiltration or through incorporation into the carbon matrix of ceramic additives via infiltration with colloidal ceramics and their subsequent conversion to refractory materials.
Carbon-carbon brake disc friction performance is dictated by the carbon microstructure which arises from the manner in which the brake disc is manufactured. The amount of graphitization, for instance, can dramatically affect frictional and wear properties. Overall brake performance is particularly affected by the individual components, including fibers and types of matrix materials, at the friction surface.
One source of problems with these carbon composites is that they have low resistance to oxidation, by atmospheric oxygen, at elevated temperatures, that is, temperatures of 500° C. (932° F.) or higher. Oxidation not only attacks the surface of the carbon-carbon composites but also enters pores that invariably are present in such structures and oxidizes the carbon fibers adjacent to the pores and surfaces of the pores, thereby weakening the composites.
Exterior surfaces of carbon-carbon composites are therefore sometimes coated with a ceramic material such as silicon carbide to prevent entry of oxidizing agents such, as molecular or ionic oxygen from the atmosphere, into the carbon-carbon composites. Silicon carbide and other antioxidant coatings are described in detail in U.S. Pat. No. 4,837,073. The exterior surfaces of carbon-carbon composites may be, alternatively, coated with a glass-forming seal coat such as a boron or boron/zirconium substance. Borate glasses have also been used from the protection of carbon-carbon composites against oxidation. U.S. Pat. No. 5,208,099 describes antioxidant coatings that are formed from a SiO2—B2O3 gel and/or sol having a SiO2:B2O3 molar composition of 60-85:40-15. Borate glass antioxidant compositions are moisture-resistant and oxidation-resistant coatings composed of 40-80 weight-% B2O3, 5-30 weight-% SiO2, 7-20 weight-% Li2O, and 7-10 weight-% ZrO2 are described in detail in U.S. Pat. No. 5,298,311.
U.S. patent application Ser. No. 09/518,013 (Golecki), filed 3 Mar. 2000, relates to carbon fiber or C—C composites that are useful in a variety of applications. Golecki teaches methods of protecting such composites against oxidation by coating them with fluidized-glass type mixtures. The fluidized-glass mixtures are maintained as liquid precursors and are applied to components formed of carbon fiber or C—C composites. Once coated with the precursors, the coated C—C components are heat-treated or annealed for one or more cycles through a series of gradual heating and cooling steps. This creates glass coatings having thicknesses of about 1-10 mils. The thicknesses of the glass coatings may be varied by varying the composition of the fluidized glass precursor mixtures, the number of application cycles, and/or the annealing parameters.
The Golecki application teaches that the fluidized glass materials may comprise such materials as borate glasses (boron oxides), phosphate glasses (phosphorus oxides), silicate glasses (silicon oxides), and plumbate glasses (lead oxides). These glasses may include phosphates of manganese, nickel, vanadium, aluminum, and zinc, and/or alkaline and alkaline earth metals such as lithium, sodium, potassium, rubidium, magnesium, and calcium and their oxides, and elemental boron and/or boron compounds such as BN, B4C, B2O3, and H3BO3. By way of example, Golecki discloses a boron-containing liquid fluidized glass precursor mixture that includes 29 weight-% phosphoric acid, 2 weight-% manganese phosphate, 3 weight-% potassium hydroxide, 1 weight-% boron nitride, 10 weight-% boron, and 55 weight-% water.
U.S. patent application Ser. No. 09/504,414 (Walker and Booker), filed 18 Feb. 2000, likewise relates to carbon-carbon composites and graphitic materials. The Walker and Booker application has as objectives the protection of carbon-carbon composites or graphites at elevated temperatures up to and exceeding 850° C. (1562° F.) and the reduction of catalytic oxidation at normal operating temperatures. Walker and Booker achieve these objectives by employing a penetrant salt solution which contains ions formed from 10-80 wt % H2O, 20-70 wt % H3PO4, 0.1-25 wt % alkali metal mono-, di-, or tri-basic phosphate, and up to 2 wt % B2O3. Their penetrant salt solutions also include at least one of MnHPO4.1.6H2O, Al(H2PO4)3, and Zn3(PO4)2, in weight-percentages up to 25 wt %, 30 wt %, and 10 wt %, respectively.